1. Field of the Invention
The present invention relates to the field of detecting flaws and discontinuities in materials, and more particularly, to the detection of flaws and discontinuities in materials using magneto-optic visualization.
2. Art Background
In many scientific, engineering and manufacturing applications, near surface cracks, voids, discontinuities and flaws in ferrous or nonferrous materials must be detected in order to insure the structural integrity of a material. For example, the material integrity of components comprising many air and space vehicles is critical for their proper operation, especially with regard to high stress components such as turbine and fan blades, rocket engine systems, air frames, etc.
A number of techniques have been developed and utilized in order to detect cracks, flaws, or the like, in materials. For example, magnetic particle methods have been employed which utilize static or "low" frequency (less than 100 Hz) magnetic fields having field components parallel to the surface of ferrous alloys which are induced by currents paralleling these surfaces. The parallel surface currents in turn may be induced, either directly, by contact electrodes, or indirectly, using coils surrounding the target material and low frequency excitation. Magnetic fields paralleling the surfaces of the target material are distorted by cracks or near surface flaws and these distortions may be detected through the use of a magnetic powder deposited on the material. Various types of magnetic powders have been developed for the visualization of sub-surface flaws. Each magnetic particle in thse powders typically consists of a single magnetic domain (i.e. a region of essentially uniform magnetization). When the magnetic powder is applied dry or in a wet slurry to a target material where a crack or flaw is present, the magnetic particles tend to aggregate and form a bridge in regions of field non-uniformities which are associated with the flaw. By mixing various pigments, flourescent dyes, and the like, with the magnetic powder, the cracks or flaws are rendered visible.
Although the magnetic powder technique is widely employed, it is a dirty and time consuming method which requies the induction of large surface currents in the material under study. Magnetic particle methods are best suited for use with low frequencies and ferrous alloys. The large masses of magnetic particles required renders the magnetic powder techniques ineffective when high frequencies are used.
Another method which has been utilized in order to detect flaws or cracks in materials is the "eddy" current technique. Eddy current techniques typically utilize a time varying electro-magnetic field which is applied to the target material being examined. Non-contact coils may then be used to excite eddy currents in the target material, such that these currents tend to flow around flaws and result in field distortions which allow the flaw to be detected in a number of well known ways. For example, circuit parameters characterizing the mutual interaction between the exciting coil and the responding material may comprise the parameters of capacitance, inductance or reactance. However, eddy current techniques require a considerable amount of support equipment and most.techniques do not result in a flaw image but rather produce data from which flaw information can be obtained only after appropriate analysis has been completed.
As will be described, the present invention provides a method for the direct optical visualization of surface and near surface cracks, flaws, etc. in ferrous and nonferrous materials. The present invention provides direct visualization of both the static and/or dynamic magnetic fields associated with the various flaws or other discontinuities in a target material, and overcomes the disadvantages associated with prior art material flaw imaging methods. In addition, the present invention is compatible with the requirements of eddy current methods while producing images of flaws directly, without the additional support equipment and data analysis required by most eddy current systems.